Current guidance systems assume that guidance lines created with GPS are a true reflection of the actual planting lines. Reducing error from GPS drift by adding options such as Real-time kinematic (RTK) correction will help create a very repeatable guidance line, but does not take into account the small errors that occur during the planting or application process. If there is an accidental shift during planting, the guidance line will indicate the rows were planted straight, but the plants will emerge shifted.
Additionally, GPS units may track slightly differently between GPS units creating a slightly different guidance line placement between planter and application equipment. Each of these instances of variation from the calculated guidance line would be corrected by a sensor controlled guidance line which responds to the plant location rather than assume planted location.
Many sidedress applicators know of these issues and choose to manually guide the equipment through the field rather than let an error in GPS guidance destroy a large section of their plants. The problem with this is human error can also cause incidental drift which will also lead to damage of plants.
U.S. Pat. No. 5,585,626 divulges the idea of using light sensors for guidance assistance, but does not provide any details on how to determine actual row location.
Current systems calculate cross track error based on where it is assumed the rows will be at. Using light sensors, the cross track error will be created from accurate measurements of the physical location of the plant row.